This is a very popular question when discussing “The Cask of Amontillado” by Edgar Allan Poe in graduate school. Cause-and-effect underscores the first –person point of view. Cause-and-effect to some degree has to be based on a reader’s almost guesswork or assumptions that there is logic in the sequence of events, in the characters’ motivations, and in the ability of the characters to do what they say they do and in the ability of the characters to say what they do. Is it possible for Montressor to have mixed the material and it still remains wet when he returns quite a while later to put his plan into motion? Is it even remotely believable that it would still be wet after hours have gone by?
A reader has to assume that Montressor is leading Fortunato into the catacombs for a reason. In searching for an answer in the story, we notice the cough, when Montressor agrees that Fortunato will not die of a cough, we either disregard that possibility or believe the narrator to be a liar. Or, we can keep both possibilities in mind, if we notice the trowel and take it seriously, we have an answer. In one sense, first-person point of view and cause-and-effect seem to depend on logic, or rationality—how could one guess what will happen next in a wholly un-rational universe? In what we can call the non-Western literary tradition, however, cause-and-effect may derive from the expectation and possibility of sheer wonder, as in the magic and sorcery of Arabian tales.
Poe laid down the rules for the short story, emphasizing that every detail should work to create a single effect. “In the whole composition,” he said in his famous review of Nathaniel Hawthorne’s Twice-Told Tales, “there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design.” In Poe’s most perfect story, every word can be analyzed with reference to it contribution to the “one pre-established design.” The following is third paragraph of “The Cask of Amontillado.”
He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity to practise imposture upon the British and Austrian MILLIONAIRES. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere….I was skilful in the Italian vintages myself, and bought largely whenever I could.
This paragraph conveys two ideas. One is that Montresor is not Italian but French. He distances himself from Italians by disparaging their judgment in "painting and gemmary." His family may have lived in Venice for several centuries, but he is still an outsider as far as upper-class Italians are concerned. His catacombs may be full of human bones--but these are not necessarily all bones of his ancestors. In fact, it is quite possible that Montresor doesn't even own his palazzo but is renting. He is obviously not affluent. “I…bought largely whenever I could” should be interpreted to mean “whenever I could afford to.”
The other idea is that both Montresor and Fortunato are specialists in luxury goods and earn their livings selling expensive merchandise to wealthy clientele. They are competitors. Montresor is poor, Fortunato is rich. It is very likely that the thousand unspecified injuries were suffered in business deals. Fortunato can outbid Montresor. Fortunato can afford to buy in larger quantities. And Fortunato has family connections going back for a thousand years. If there is something good to be bought at a bargain price, he is more likely to hear about it first.
When Montresor says he has bought a cask of Amontillado, Fortunato says, "Impossible!" What he really means is that it would be impossible for Montresor to learn about such a valuable cargo before he did. But this is carnival season. Fortunato has been drinking, not attending to business. He thinks this is why Montresor has gotten ahead of him for once. However, Montresor has only bought one "pipe" (126 gallons) because, as he says, "I have my doubts." Montresor would have bought more if he had been sure of its quality. Fortunato is highly motivated to taste it--not because he needs to consume any more wine, not because he is anxious to show off his connoisseurship, not to accommodate a friend--but because he wants to buy some of the Amontillado himself for resale. But he too must judge its quality. He can afford to buy the whole shipload and make a big profit--and Montresor knows that is exactly what he is planning to do, because that is exactly the sort of thing Fortunato has done to him in the past. Montresor knows that Fortunato is already thinking of tasting the Amontillado, pronouncing it ordinary sherry, and then going off to find the ship that recently brought it into port.
I assume you're interested in what caused Montresor to hate Fortunato so fiercely that he wanted to murder him. If you read the third paragraph in the story you will observe that the two men appear to be competitors in trading in luxury goods. Montresor says, "I was skilful in the Italian vintages myself, and bought largely whenever I could." That means whenever he could afford to buy largely, because Italian vintages would always be available. He always had a problem of what he could afford.
Fortunato is rich and Montresor is poor. This gives Fortunato the ability to make better buys because he can buy in larger quantities and pay cash. It is in dealings of these kinds that he is able to inflict the "thousand injuries" which have aroused such hatred in Montresor, who, as his name suggests, is an outsider, a Frenchman.
Fortunato not only has the money to buy up any good merchandise that becomes available, but he has connections enabling him to learn about such things as estate sales, arrivals of ships carrying luxury goods, and financial setbacks of individuals forcing liquidation of art works, "gemmary," and fine furnishings.
When Montresor tells him he has just purchased a cask of Amontillado, Fortunato says, "Impossible!" He means that he considers it impossible that Montresor should have heard about the shipment before he did. There is a lot of money to be made from a whole shipload of Amontillado, but Fortunato must first make sure it is genuine. That is his real motive for insisting on tasting the wine that very night. He is probably planning on buying up the entire cargo, thereby preventing Montresor from obtaining any more Amontillado, and inflicting yet another "injury." It is not just a question of drinking a glass of sherry deep underground in a poisonous atmosphere, or of showing off his connoisseurship; he is thinking about making money. The Amontillado is merchandise.
I suggest that you re-read the third paragraph of the story carefully in order to understand the cause of Montresor's hatred and of Fortunato's death. Montresor not only satisfies his desire for revenge, but he gets rid of a competitor who has prevented him from making numerous profitable investments.
The "thousand injuries of Fortunato" cause Montresor to hate him and plan to kill him. The effect of the thousand injuries is described in the story. He is lured into the catacombs, chained to the rock wall, and left to die of slow starvation.